[Farming Zanuka Hunter, help needed to understand correctly.]

Addressing you questions in order:

1) To my knowledge, there isn't a specific mission to make field bosses appear that's more likely than others.

2) Yes, marks disappear once you kill the assassin, or they kill you. Very grindy

3) I think it has to be 5 different invasion missions, not certain.

4) Check their profiles is the only way I know.

[Story Speculation and Feedback (Spoilers, of course)]

Certain aspects of the quest did bring up some questions for me in regards to the story.

Spoiler

The Orokin ciphers were a fun take on hacking that gave more depth to their civilization, but the markings we looked for were stated to be fresh. Who or what made them?

Assuming that the lab was where Ballas was hiding, why leave the vitruvian (besides to advance plot)? If umbra was in the lab was it just in suspended animation until the aftershocks damaged something? Alternatively, if Ballas wasn't in the moon the whole time, has he just been hiding in the Tau system? Similar question, if umbra wasn't in cryostasis then where was he this whole time?

Personally, I figure that it was likely Lotus or rather (borrowing the cephelon term) her leftover precepts that has been trying to reach out to us and giving hints for the cipher to the lab. Assuming from info from the Natah quest, that she was avoiding the Sentients to avoid being brainwashed back into them, her helmet and wires might have been acting as a shield for her sense of self. There's a small piece of her that wants to protect us, but doesn't have much control over the main body that's been reprogrammed by the sentient hivemind.

 

Based on what we've seen in regards to Excalibur Umbra, I'm going to guess that if there are other umbra frames that they will have the independent action passive as well as specialized towards fighting Sentients but in a low key manner. They'll have one of their warframe abilities modified to be able to strip Sentient resistances. Lore-wise, umbras would effectively be the prototypes of the warframe-infestation strains used from Dax soldiers and the likes as the base. The primes were likely copies of the umbras made after they were "tamed" and had less sense of self.